1. Field of the Invention
This invention relates to improvements in or relating to bone fracture fixation plates and related devices.
2. Description of the Background Art
Fixation of bone fractures in humans is commonly achieved by the use of a plaster cast to prevent undue relative movement of the fracture ends of the bone. With the cast in place, the fractured bone reunites with a large amount of callus being formed about the fracture site. After the plaster cast is removed, the reunited bone is initially allowed only a limited load bearing function whereafter there is a gradual return to the normal load bearing function.
However, with the use of plaster casts, there is difficulty in obtaining adequate reduction of the fracture and there is a tendency for the joints on either side of the fracture to stiffen and for the muscle surrounded by the cast to atrophy. In addition, this method of treatment is contraindicated for use with open fractures. Within recent decades there has therefore developed significant use of an alternative fracture fixation method, internal fixation, in which a rigid plate is secured to the fractured bone to span the fracture thereby aligning the fractured ends of the bone for union and removing the load bearing function from the fractured section of the bone.
The major efforts in the development of internal fixation have been towards a rigid fixation whereby a bone fracture fixation plate or internal fixation plate of a rigid material such as stainless steel is securely attached to the bone to hold the fractured ends together without relative movement. This approach has the great advantage that the bone can be brought under normal load very quickly, thus aiding the healing process and greatly reducing the period of incapacitation of the patient.
This rigid fixation with a metal plate with mechanical stiffness in excess of that of the bone results in a primary end-to-end union of the fractured ends with little or no callus formation. However, the healing process is normally accelerated by moderate flexure of the bone about the fracture site and consequently with such rigid fixing methods primary union in humans is slow, taking about 12 to 24 months. During such a prolonged period of healing and after the fracture has been united the bone shielded from strains (stresses) by the plates may undergo osteopenia, i.e. bone porosity may increase and cortical thickness may decrease, which may result in refracture following removal of the plate. The shielding of bone from strain (stress) as a result of rigid fixation is termed stress shielding. Alternatively, if fractured bone ends fixed by conventional metal plates are separated by some gap, their motion under repeated loading may lead to sudden plate fatigue failure during the healing period, disrupting the healing process.
Furthermore, conventional metal plates are somewhat disadvantageous as they may release undesirable metal ions and can adversely affect cells, tissues and organs, particularly over the long healing periods mentioned above. They therefore normally require removal in a second surgical operation as soon as is feasible.
Alternative methods of fixation employ metal rods (pins) inserted into the medullary canal of the fractured long bone to achieve alignment of the fractured bone ends. Disadvantages of this include metal ion release and a bending rigidity that may shield surrounding bone from stress once the fracture is united.
In an attempt to avoid these problems, there have been developed plates, generally glass or carbon fibre reinforced polymer composites, of lower rigidity than conventional stainless steel plates. Such plates however have not so far proved entirely satisfactory since it is difficult to attain a balance between the relatively high rigidity of fixture of the fractured ends required in the initial stages of healing and the lower degree of rigidity of fixture required in the later stages of healing to promote the healing process and avoid stress shielding and osteopenia. On the one hand there is a danger that these polymer plates, which have rigidities closer to that of the bone than that of conventional metal plates, will not have sufficient strength to support applied loads before the fracture heals sufficiently to provide some rigidity in the bone itself. On the other hand if the plates are more rigid than the reunited bone there is still the danger of stress shielding; because different bones in different individuals vary in stiffness it will never be possible to fabricate a plate that exactly matches the stiffness of a particular bone.
Recently, there have been investigations into the use of absorbable materials as internal fixation plates. These plates rely on the dissolution or resorption of the plates within the body so that by the time the fracture is healed little or none of the original plate remains. Thus for example Tunc et al (Proc. 9th Int. Meeting of the Soc. for Biomaterials (1983) 47) have disclosed the use of plates comprising totally absorbable high molecular weight polylactide polymer. Alexander et al. (Trans. 11th Int'l Biomat. Symp. 11 (1979)) have disclosed plates of absorbable polylactic acid polymer reinforced with carbon fibres. Parsons et al. (5th Annual Meeting of the Society for Biomaterials (1979)) have disclosed plates comprising absorbable polylactic acid polymer reinforced with continuous carbon fibres. Vert et al. (U.S. Pat. No. 4,279,249) have disclosed plates of absorbable polylactic acid polymer reinforced with absorbable polyglycolic acid polymer.
However, the biological response which results in the resorption of such plates in the body creates further problems. Thus this biological response may not be sufficiently specific to resorb only the plates and can also cause destruction of the underlying bone and of soft tissue in the vicinity. Furthermore, substances released by the resorption of the plates may accumulate at such sites as lymph nodes and cause adverse tissue changes. Moreover, non-resorbable fibre reinforcement material released from the resorbable matrix of a fibre reinforced resorbable plate may be dispersed throughout the tissue of the patient.
A further line of approach has been to use composite systems comprising rigid and resorbable components in which, on resorption of the resorbable component, the rigid component is freed of its load bearing function. This can occur suddenly as with the fracture clamp of U.S. Pat. No. 2,987,062 which comprises two metallic bands directly connected at one pair of ends and joined by absorbable catgut at the other pair of ends. Loss of rigidity can also occur gradually as with the plates of U.S. Pat. No. 4,338,926 in which a rigid plate secured by screws to the fractured bone is provided with a resorbable element lying between the rigid plate and the bone or the securing screws, upon the resorption of which element the attachment of the rigid plate is loosened and its load bearing function is lost. In the case where the composite system looses its load bearing function suddenly, there are on the one hand the obvious dangers of system failure before fracture reunion is completed thus subjecting the incompletely healed bone to the full stress of normal load bearing and on the other hand the dangers of osteopenia if the composite system does not yield its load bearing function until a relatively long time after fracture reunion is completed. In the second case, as with the resorbable plates discussed above, significant amounts of resorbable material are released into the body by the composite system will all the possible dangers that that entails.